Kathrin Freyler

Load Dependency of Postural Control--Kinematic and Neuromuscular Changes in Response to over and under Load Conditions.

Introduction Load variation is associated with changes in joint torque and compensatory reflex activation and thus, has a considerable impact on balance control. Previous studies dealing with over (OL) and under loading (UL) used water buoyancy or additional weight with the side effects of increased friction and inertia, resulting in substantially modified test paradigms. The purpose of this study was to identify gravity-induced load dependency of postural control in comparable experimental conditions and to determine the underlying neuromuscular mechanisms. Methods Balance performance was recorded under normal loading (NL, 1g), UL (0.16g; 0.38g) and OL (1.8g) in monopedal stance. Center of pressure (COP) displacement and frequency distribution (low 0.15-0.5Hz (LF), medium 0.5-2Hz (MF), high 2-6Hz (HF)) as well as ankle, knee and hip joint kinematics were assessed. Soleus spinal excitability was determined by H/M-recruitment curves (H/M-ratios). Results Compared to NL, OL caused an increase in ankle joint excursion, COP HF domain and H/M-ratio. Concomitantly, hip joint excursion and COP LF decreased. Compared to NL, UL caused modulations in the opposite direction: UL decreased ankle joint excursions, COP HF and H/M-ratio. Collaterally, hip joint excursion and COP LF increased. COP was augmented both in UL and in OL compared to NL. Conclusion Subjects achieved postural stability in OL and UL with greater difficulty compared to NL. Reduced postural control was accompanied by modified balance strategies and compensatory reflex activation. With increasing load, a shift from hip to ankle strategy was observed. Accompanying, COP frequency distribution shifted from LF to HF and spinal excitability was enhanced. It is suggested that in OL, augmented ankle joint torques are compensated by quick reflex-induced postural reactions in distal muscles. Contrarily, UL is associated with diminished joint torques and thus, postural equilibrium may be controlled by the proximal segments to adjust the center of gravity above the base of support.

Improved postural control in response to a 4-week balance training with partially unloaded bodyweight

Balance training (BT) is successfully implemented in therapy as a countermeasure against postural dysfunctions. However, patients suffering from motor impairments may not be able to perform balance rehabilitation with full body load. The purpose of this study was to investigate whether partial unloading leads to the same functional and neuromuscular adaptations. The impact on postural control of a 4-week BT intervention has been compared between full and partial body load. 32 subjects were randomly assigned to a CON (conventional BT) or a PART group (partially unloaded BT). BT comprised balance exercises addressing dynamic stabilization in mono- and bipedal stance. Before and after training, centre of pressure (COP) displacement and electromyographic activity of selected muscles were monitored during different balance tasks. Co-contraction index (CCI) of soleus (SOL)/tibialis (TA) was calculated. SOL H-reflexes were elicited to evaluate changes in the excitability of the spinal reflex circuitry. Adaptations in response to the training were in a similar extent for both groups: (i) after the intervention, the COP displacement was reduced (P<0.05). This reduction was accompanied by (ii) a decreased CCI of SOL/TA (P<0.05) and (iii) a decrease in H-reflex amplitude (P<0.05). BT under partial unloading led to reduced COP displacements comparable to conventional BT indicating improved balance control. Moreover, decreased co-contraction of antagonistic muscles and reduced spinal excitability of the SOL motoneuron pool point towards changed postural control strategies generally observed after full body load training. Thus, BT considering partial unloading is an appropriate alternative for patients unable to conduct BT under full body load.

Balance impairments and neuromuscular changes in breast cancer patients with chemotherapy-induced peripheral neuropathy

OBJECTIVE Chemotherapy-induced peripheral neuropathy (CIPN) is a common side effect of cancer treatment. Resulting sensory and motor dysfunctions often lead to functional impairments like gait or balance disorders. As the underlying neuromuscular mechanisms are not fully understood, we compared balance performance of CIPN patients with healthy controls (CON) to specify differences responsible for postural instability. METHODS 20 breast cancer patients with CIPN (PAT) and 16 matched CONs were monitored regarding centre of pressure displacement (COP) and electromyographic activity of M. soleus, gastrocnemius, tibialis anterior, rectus femoris and biceps femoris. We calculated antagonistic co-contraction indices (CCI) and elicited soleus H-reflexes to evaluate changes in the elicitability and sensitivity of spinal reflex circuitry. RESULTS PATs COP displacement was greater than CONs (p=.013) and correlated significantly with the level of CCIs and self-reported CIPN symptoms. PAT revealed prolonged H-wave latency (p=.021), decreased H-reflex elicitability (p=.001), and increased H-reflex sensitivity from bi- to monopedal stance (p=.004). CONCLUSIONS We summarise that CIPN causes balance impairments and leads to changes in elicitability and sensitivity of spinal reflex circuitry associated with postural instability. We assume that increased simultaneous antagonistic muscle activation may be used as a safety strategy for joint stiffness to compensate for neuromuscular degradation. SIGNIFICANCE Sensorimotor training has the potential to influence neuromuscular mechanisms in order to improve balance performance. Therefore, this training modality should be evaluated as a possible treatment strategy for CIPN.

Reactive Balance Control in Response to Perturbation in Unilateral Stance: Interaction Effects of Direction, Displacement and Velocity on Compensatory Neuromuscular and Kinematic Responses.

Unexpected sudden perturbations challenge postural equilibrium and require reactive compensation. This study aimed to assess interaction effects of the direction, displacement and velocity of perturbations on electromyographic (EMG) activity, centre of pressure (COP) displacement and joint kinematics to detect neuromuscular characteristics (phasic and segmental) and kinematic strategies of compensatory reactions in an unilateral balance paradigm. In 20 subjects, COP displacement and velocity, ankle, knee and hip joint excursions and EMG during short (SLR), medium (MLR) and long latency response (LLR) of four shank and five thigh muscles were analysed during random surface translations varying in direction (anterior-posterior (sagittal plane), medial-lateral (frontal plane)), displacement (2 vs. 3cm) and velocity (0.11 vs. 0.18m/s) of perturbation when balancing on one leg on a movable platform. Phases: SLR and MLR were scaled to increased velocity (P<0.05); LLR was scaled to increased displacement (P<0.05). Segments: phasic interrelationships were accompanied by segmental distinctions: distal muscles were used for fast compensation in SLR (P<0.05) and proximal muscles to stabilise in LLR (P<0.05). Kinematics: ankle joints compensated for both increasing displacement and velocity in all directions (P<0.05), whereas knee joint deflections were particularly sensitive to increasing displacement in the sagittal (P<0.05) and hip joint deflections to increasing velocity in the frontal plane (P<0.05). COP measures increased with increasing perturbation velocity and displacement (P<0.05). Interaction effects indicate that compensatory responses are based on complex processes, including different postural strategies characterised by phasic and segmental specifications, precisely adjusted to the type of balance disturbance. To regain balance after surface translation, muscles of the distal segment govern the quick regain of equilibrium; the muscles of the proximal limb serve as delayed stabilisers after a balance disturbance. Further, a kinematic distinction regarding the compensation for balance disturbance indicated different plane- and segment-specific sensitivities with respect to the determinants displacement and velocity.

Specific Stimuli Induce Specific Adaptations: Sensorimotor Training vs. Reactive Balance Training

Typically, balance training has been used as an intervention paradigm either as static or as reactive balance training. Possible differences in functional outcomes between the two modalities have not been profoundly studied. The objective of the study was to investigate the specificity of neuromuscular adaptations in response to two balance intervention modalities within test and intervention paradigms containing characteristics of both profiles: classical sensorimotor training (SMT) referring to a static ledger pivoting around the ankle joint vs. reactive balance training (RBT) using externally applied perturbations to deteriorate body equilibrium. Thirty-eight subjects were assigned to either SMT or RBT. Before and after four weeks of intervention training, postural sway and electromyographic activities of shank and thigh muscles were recorded and co-contraction indices (CCI) were calculated. We argue that specificity of training interventions could be transferred into corresponding test settings containing properties of SMT and RBT, respectively. The results revealed that i) postural sway was reduced in both intervention groups in all test paradigms; magnitude of changes and effect sizes differed dependent on the paradigm: when training and paradigm coincided most, effects were augmented (P<0.05). ii) These specificities were accompanied by segmental modulations in the amount of CCI, with a greater reduction within the CCI of thigh muscles after RBT compared to the shank muscles after SMT (P<0.05). The results clearly indicate the relationship between test and intervention specificity in balance performance. Hence, specific training modalities of postural control cause multi-segmental and context-specific adaptations, depending upon the characteristics of the trained postural strategy. In relation to fall prevention, perturbation training could serve as an extension to SMT to include the proximal segment, and thus the control of structures near to the body’s centre of mass, into training.

Posture and Locomotion

Gravity affects the human body in numerous ways. This chapter reviews recent findings on how the nervous system governs muscle forces to control upright posture and locomotion in varying gravity conditions. With an emphasis on gravity conditions below Earth gravitation, three major aspects for the control of stance and gait are presented for short-term and long-term adaptation: the integration of sensory feedback via spinal and supraspinal circuitries to command the neuromuscular system governing the movement and the biomechanical output which defines the quality of these motor skills. Numerous experiments executed in space flight or simulation studies frame the content of this chapter that contains the sub-themes: posture control and locomotion.

Alleviation of Motor Impairments in Patients with Cerebral Palsy: Acute Effects of Whole-body Vibration on Stretch Reflex Response, Voluntary Muscle Activation and Mobility

Introduction Individuals suffering from cerebral palsy (CP) often have involuntary, reflex-evoked muscle activity resulting in spastic hyperreflexia. Whole-body vibration (WBV) has been demonstrated to reduce reflex activity in healthy subjects, but evidence in CP patients is still limited. Therefore, this study aimed to establish the acute neuromuscular and kinematic effects of WBV in subjects with spastic CP. Methods 44 children with spastic CP were tested on neuromuscular activation and kinematics before and immediately after a 1-min bout of WBV (16–25 Hz, 1.5–3 mm). Assessment included (1) recordings of stretch reflex (SR) activity of the triceps surae, (2) electromyography (EMG) measurements of maximal voluntary muscle activation of lower limb muscles, and (3) neuromuscular activation during active range of motion (aROM). We recorded EMG of m. soleus (SOL), m. gastrocnemius medialis (GM), m. tibialis anterior, m. vastus medialis, m. rectus femoris, and m. biceps femoris. Angular excursion was recorded by goniometry of the ankle and knee joint. Results After WBV, (1) SOL SRs were decreased (p < 0.01) while (2) maximal voluntary activation (p < 0.05) and (3) angular excursion in the knee joint (p < 0.01) were significantly increased. No changes could be observed for GM SR amplitudes or ankle joint excursion. Neuromuscular coordination expressed by greater agonist–antagonist ratios during aROM was significantly enhanced (p < 0.05). Discussion The findings point toward acute neuromuscular and kinematic effects following one bout of WBV. Protocols demonstrate that pathological reflex responses are reduced (spinal level), while the execution of voluntary movement (supraspinal level) is improved in regards to kinematic and neuromuscular control. This facilitation of muscle and joint control is probably due to a reduction of spasticity-associated spinal excitability in favor of giving access for greater supraspinal input during voluntary motor control.

Acute whole-body vibration increases reciprocal inhibition

Based on previous evidence that whole-body vibration (WBV) affects pathways involved in disynaptic reciprocal inhibition (DRI), the present hypothesis-driven experiment aimed to assess the acute effects of WBV on DRI and co-contraction. DRI from ankle dorsiflexors to plantar flexors was investigated during submaximal dorsiflexion before and after 1 min of WBV. With electromyography, musculus soleus (SOL) H-reflex depression following a conditioning stimulation of the peroneal nerve (1.1x motor threshold for the musculus tibialis anterior, TA) was assessed and co-contraction was calculated. After WBV, DRI was significantly increased (+4%, p < 0.05). SOL (-13%, p < 0.05) and TA (-6%, p < 0.05) activities were significantly reduced; co-contraction tended to be diminished (-8%, p = 0.05). Dorsiflexion torque remained unchanged. After WBV, DRI increased during submaximal isometric contraction in healthy subjects. The simultaneous SOL relaxation and TA contraction indicate that a more economic movement execution is of functional significance for WBV application in clinical and athletic treatment.

Neuromuscular and Kinematic Adaptation in Response to Reactive Balance Training – a Randomized Controlled Study Regarding Fall Prevention

Slips and stumbles are main causes of falls and result in serious injuries. Balance training is widely applied for preventing falls across the lifespan. Subdivided into two main intervention types, biomechanical characteristics differ amongst balance interventions tailored to counteract falls: conventional balance training (CBT) referring to a balance task with a static ledger pivoting around the ankle joint versus reactive balance training (RBT) using externally applied perturbations to deteriorate body equilibrium. This study aimed to evaluate the efficacy of reactive, slip-simulating RBT compared to CBT in regard to fall prevention and to detect neuromuscular and kinematic dependencies. In a randomized controlled trial, 38 participants were randomly allocated either to CBT or RBT. To simulate stumbling scenarios, postural responses were assessed to posterior translations in gait and stance perturbation before and after 4 weeks of training. Surface electromyography during short- (SLR), medium- (MLR), and long-latency response of shank and thigh muscles as well as ankle, knee, and hip joint kinematics (amplitudes and velocities) were recorded. Both training modalities revealed reduced angular velocity in the ankle joint (P < 0.05) accompanied by increased shank muscle activity in SLR (P < 0.05) during marching in place perturbation. During stance perturbation and marching in place perturbation, hip angular velocity was decreased after RBT (P from TTEST, Pt < 0.05) accompanied by enhanced thigh muscle activity (SLR, MLR) after both trainings (P < 0.05). Effect sizes were larger for the RBT-group during stance perturbation. Thus, both interventions revealed modified stabilization strategies for reactive balance recovery after surface translations. Characterized by enhanced reflex activity in the leg muscles antagonizing the surface translations, balance training is associated with improved neuromuscular timing and accuracy being relevant for postural control. This may result in more efficient segmental stabilization during fall risk situations, independent of the intervention modality. More pronounced modulations and higher effect sizes after RBT in stance perturbation point toward specificity of training adaptations, with an emphasis on the proximal body segment for RBT. Outcomes underline the benefits of balance training with a clear distinction between RBT and CBT being relevant for training application over the lifespan.

Changes in balance strategy and neuromuscular control during a fatiguing balance task: A study in perturbed unilateral stance

Fatigue impairs sensorimotor performance, reduces spinal reflexes and affects the interaction of antagonistic muscles in complex motor tasks. Although there is literature dealing with the interference of fatigue and postural control, the interpretation is confounded by the variety of paradigms used to study it. This study aimed to evaluate the effects of postural fatigue on balance control and strategy, as well as on neuromuscular modulation, in response to postural perturbation (PERT) during a fatiguing balance task. A fatigue protocol consisting of continuous exposure to perturbations until exhaustion was executed in 24 subjects. Number of failed attempts, paths of center of pressure displacement (COP), ankle, knee, and hip joint kinematics, electromyographic activity of the soleus (SOL), tibialis anterior (TA), rectus femoris (RF), vastus lateralis (VL), biceps femoris (BF), and gluteus maximus muscles (GM) and spinal excitability of SOL at the peak of the short-latency responses (SLR) were recorded after posterior PERT. The co-contraction index (CCI) was calculated for TA_SOL, VL_BF and RF_GM. (1) The number of failed attempts significantly increased while COP amplitude and velocity, as well as angular excursion at the ankle, knee and hip joints, decreased with fatigue (P < 0.05). (2) Concomitantly, CCI of SOL_TA, VL_BF and RF_GM increased and spinal excitability in SOL declined. (3) Adaptations progressively augmented with progressing exhaustion and occurred in the distal prior to proximal segment. Distinctly deteriorated balance ability was accompanied by a modified neuromuscular control—the increase in co-contraction reflected by simultaneously activated antagonists is accompanied by smaller knee and hip joint excursions, indicating an elevated level of articular stiffness. These changes may be associated with an exaggerated postural rigidity and could have caused the delayed and reduced postural reactions that are reflected in the changes in COP displacement when compensating for sudden PERT. The reduction in spinal excitability may either be caused by fatigue itself or by an increase in reciprocal inhibition due to augmented TA activity.